nfsd: degraded slot-count more gracefully as allocation nears exhaustion. (2030ca56) · Commits · 戴 / test

This original code in nfsd4_get_drc_mem() would hand out 30 slots (approximately NFSD_MAX_MEM_PER_SESSION bytes at slightly over 2K per slot) to each requesting client until it ran out of space, then it would possibly give one last client a reduced allocation, then fail the allocation. Since commit ("nfsd: give out fewer session slots as limit approaches") the last 90 slots to be given to about 12 clients with quickly reducing slot counts (better than just 3 clients). This still seems unnecessarily hasty. A subsequent patch allows over-allocation so every client gets at least one slot, but that might be a bit restrictive. The requested number of nfsd threads is the best guide we have to the expected number of clients, so use that - if it is at least 8. 256 threads on a 256Meg machine - which is a lot for a tiny machine - would result in nfsd_drc_max_mem being 2Meg, so 8K (3 slots) would be available for the first client, and over 200 clients would get more than 1 slot. So I don't think this change will be too debilitating on poorly configured machines, though it does mean that a sensible configuration is a little more important. Signed-off-by:

NeilBrown <neilb@suse.de> Signed-off-by:

J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

+11 −4

Original line number	Diff line number	Diff line
		@@ -1568,11 +1568,12 @@ static inline u32 slot_bytes(struct nfsd4_channel_attrs *ca)
		* re-negotiate active sessions and reduce their slot usage to make
		* room for new connections. For now we just fail the create session.
		*/
		static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca)
		static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs ca, struct nfsd_net nn)
		{
		u32 slotsize = slot_bytes(ca);
		u32 num = ca->maxreqs;
		unsigned long avail, total_avail;
		unsigned int scale_factor;

		spin_lock(&nfsd_drc_lock);
		if (nfsd_drc_max_mem > nfsd_drc_mem_used)
		@@ -1586,12 +1587,18 @@ static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca)
		total_avail = 0;
		avail = min((unsigned long)NFSD_MAX_MEM_PER_SESSION, total_avail);
		/*
		* Never use more than a third of the remaining memory,
		* Never use more than a fraction of the remaining memory,
		* unless it's the only way to give this client a slot.
		* The chosen fraction is either 1/8 or 1/number of threads,
		* whichever is smaller. This ensures there are adequate
		* slots to support multiple clients per thread.
		* Give the client one slot even if that would require
		* over-allocation--it is better than failure.
		*/
		avail = clamp_t(unsigned long, avail, slotsize, total_avail/3);
		scale_factor = max_t(unsigned int, 8, nn->nfsd_serv->sv_nrthreads);

		avail = clamp_t(unsigned long, avail, slotsize,
		total_avail/scale_factor);
		num = min_t(int, num, avail / slotsize);
		num = max_t(int, num, 1);
		nfsd_drc_mem_used += num * slotsize;
		@@ -3188,7 +3195,7 @@ static __be32 check_forechannel_attrs(struct nfsd4_channel_attrs *ca, struct nfs
		* Note that we always allow at least one slot, because our
		* accounting is soft and provides no guarantees either way.
		*/
		ca->maxreqs = nfsd4_get_drc_mem(ca);
		ca->maxreqs = nfsd4_get_drc_mem(ca, nn);

		return nfs_ok;
		}

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